Simpleceiver ~ Part 26
N6QW Back on the Blog (at least part time)!
A family medical emergency had caused me to stop blogging but that situation is somewhat improving and so I am now back on the keyboard. After three weeks of being turned off, I can see where my soldering iron actually has rust on the tip --that is not good and I soon hope to be soldering some parts on island squares. I might also have to recalibrate the CNC Mill.
Our plan is to now focus on the building the transmitter and then take the next step of configuring the blocks (receiver, transmitter, control and LPF + TR) into a working transceiver. During the last three weeks KK6FUT has built the DCR version and has a you tube video -- it sounds really good as does the superhet Simpleceiver built by Mikele, 9A3XZ. I do know that others are building portions of the project so if anyone has built either the DCR or superhet variant please let us know.
In moving on the transmitter, it would be well to review the final configuration. Essentially the design intent was to build completely separate receiver and transmitter boards and to share a common VFO and BFO and thus one would have a transreceiver. There are benefits as well as potential problems in this approach. The chief benefit is that you can optimize the receiver for best performance while not impacting the transmitted signal.
[Many older technology transceivers had this problem and frequently the alignment instructions would strongly suggest that the final alignment was a compromise between that which could be achieved with separate circuits. Today that is not so much a problem and it is probably for the second reason that separates were chosen and that is experimentation. With the separate approach it would be possible to test variants individually without compromising overall performance. For instance one change might be to put a 6 pole filter in the receiver and only use a 4 pole for the transmitter. Or to even have switchable filters, SSB/CW, in the receiver. You get the idea.]
One other problem rears its head especially in the older technology transceivers and that was circuit loading that caused frequency shifts when going from transmit to receive. Imagine my surprise when I was working with the Ten Tec Model 150A SSB/CW transceiver to find there was a compensating circuit in the BFO circuit so that the BFO was calibrated in receive and then using circuitry that was only switched into play on transmit was calibrated on transmit.
Being on the same frequency in both transmit and receive is critical! Typically where there are crystal oscillators involved, variable circuit loading can cause a shift in the fundamental frequency. A shift of 100 Hz can be picked off with some of the new crop SDR radios --and if you are on 40 Meters many times the newly minted Extra's running those $10 K SDR radio using a 37 inch flat screen for a waterfall display will tell you that you are 100 Hz low (or high) followed quickly by a command to get on frequency --you are on frequency it is just the BFO shifted.
My original thoughts were to share a common BFO and VFO for the receiver and transmitter. Because of what was just described I am now leaning to separate BFO's and a common VFO. The VFO if you use the AD9850 or the Si5351 is fairly immune to frequency shifts caused by circuit loading since the frequency being generated is a result of mathematics and not an inductor / capacitor. Thus the plan is to have separate BFO's that can be individually "netted" (an old term for being on the same output frequency). If the Si5351 is used it may be even a lesser problem as the BFO like wise could be generated mathematically and not subject to loading. But with the AD9850 (or LC VFO or VXO) you will have to supply a BFO signal. In my current build I am using the AD9850.
So Ok how do you get two BFO's on the same frequency and how do you switch them on and off. Having two BFO's running continuously is more of a problem on the receive side than the transmit side. If the transmit BFO was only 20 Hz off (not enough for the SDR guy to scream at you) but is sufficient to be heard in the receiver. So the plan is to leave the receive BFO run all of the time and to only run the transmit BFO when in transmit. In our earlier post on control circuits which uses a NE555 that will work nicely to power on the transmit BFO. So that part of the problem may be minimized.
The real nut to crack is the netting. Assuming you have the receiver working properly and on the correct frequency then it is a matter of a little cut and try. In my receiver build I used a socket for the BFO crystal (three pins of a SIP socket) and this functionality will provide a means of finding two crystals that will be very close in frequency. Before I started this project I had a batch of 12.096 MHz crystals and then purchased another batch of Series type crystals. My process for finding two close crystals was to tune in say a net on 40M with the normally used BFO crystal and then to simply cycle through a batch of crystals that I had. It is a pretty simple mater to find ones that are higher or lower in frequency as the pitch of the voices will change. There will be a grouping that then seems to produce the same pitched sound. Run this group through a second time and "listen" for the closest ones. The ear is quite good at this process. The ones that pass this test are candidates for the transmitter carrier oscillator.
What is significant here is that the crystals are tested in a circuit that is used in the receiver and that same circuit will be used in the transmitter. The feature of this circuit is a small trimmer cap to put the crystal on the exact frequency needed. The same value trimmer is used in the carrier oscillator. Thus it will be possible later (just like the Ten Tec Model 150A) to calibrate the transmitter carrier oscillator on the same frequency thereby bypassing the wrath of the 40M SDR Frequency Police!
I will stop here and pick up next time with more discussion of the transmitter circuits.